1. Field of the Invention
The present invention relates to a binary image reduction method which is free from degradation of image quality of an image expressed by black and white, i.e., binary values and, more particularly, to a binary image reduction method which can prevent thin lines from being lost so that an uneven thin line generated when an image is read by an image reading apparatus such as a facsimile can be prevented from being disconnected.
2. Description of the Prior Art
As a conventional binary image reduction method, an SPC (Selective Processing Conversion) method shown in FIG. 1(a), a field mapping method shown in FIG. 1(b), and the like are known.
In the SPC method, when a converted picture is mapped to an original image, a value of a pixel of the original picture (to be referred to as an original pixel hereinafter) at a position closest to a pixel of the converted picture to be obtained (to be referred to as a converted pixel hereinafter) is, assigned to the converted pixel. In FIG. 1(a), the value of an original pixel P.sub.0.0 is assigned to converted pixel Q. In the field mapping method in FIG. 1(b), four original pixels near a converted pixel are selected, an average density which is assigned to the converted pixel position by these four original pixels is calculated, and the calculated density is subjected to threshold value processing to be converted to the value of the converted pixel. In FIG. 1(b), the value of a converted pixel Q is obtained based on four original pixels P.sub.0.0, P.sub.0.1, P.sub.1.0, and P.sub.1.1.
However, in the above-mentioned reduction methods, when a black or white thin line is present on an original picture, if the thin line on the original picture is not an original pixel closest to the converted pixel position in the SPC method or if the result of the threshold value processing of the average density of the by four associated original pixels does not coincide with the value of the thin line in the field mapping method, the corresponding thin line is lost from the converted picture, resulting in problems of skipping and batter. The "skipping" occurs when a black thin line of an original picture is lost from the converted picture, and the "batter" occurs when a white thin line is lost. In FIGS. 1(a) and 1(b), the value of the converted pixel Q indicates white, and a black thin line (P.sub.1.0, P.sub.1.1) of an original picture is lost.
As a method of preventing skipping or batter caused by a lost thin line, an improved TP (Thin Line Preservation) method (Japanese Patent Application No. 62-236087, Sept. 22, 1987) has been proposed. In the TP method, a thin line on an original picture is detected, and the detected thin line is preserved on the converted picture, thus preventing skipping or batter. FIG. 2 shows the basic principle of thin line preservation. In FIG. 2, reference symbols P.sub.0.0, P.sub.0.1, P.sub.1.0, P.sub.1.1, P.sub.2.0, and P.sub.2.1 denote original pixels; and Q, a converted pixel. Assuming that (P.sub.m.m) represents a value of the original pixel, (P.sub.m.m) represents an inverse value of (P.sub.m.m), and (Q) represents a value of the converted pixel, if (P.sub.0.0)=(P.sub.2.0)=(P.sub.1.0), these pixels are detected as a thin line, and (Q)=(P.sub.1.0) is set.
When the TP method is applied to a reduction ratio of 7/8, a thin line is determined using original pixel references shown in FIG. 3. More specifically, six original pixels in each of the horizontal and vertical directions (i.e., forming a cross shape) are referred to have an original pixel closest to a target of converted pixels as a central pixel. Thin line determination conditions in FIG. 3 are as follows. That is when one of (P.sub.0.0)=(P.sub.-2.0)=(P.sub.-1.0), (P.sub.0.0) =(P.sub.2.0)=(P.sub.1.0), and (P.sub.0.0)=(P.sub.3.0)=(P.sub.1.0)=(P.sub.2.0) is established, the presence of a vertical thin line is determined. When one of (P.sub.0.0)=(P.sub.0.-2)=(P.sub.0.-1), (P.sub.0.0)=(P.sub.0.2)=(P.sub.0.1), and (P.sub.0.0)=(P.sub.0.3) =(P.sub.0.1)=(P.sub.0.2) is established, the presence of a horizontal thin line is determined. However, an image read from a scanner such as a facsimile apparatus often has an uneven pattern. In the TP method, if the thin line includes the uneven pattern, this pattern is recognized as a thin line in an opposite color and is preserved, resulting in loss of the thin line. FIGS. 4(a) and 4(b) show a case wherein since an original picture includes an uneven pattern at a reduction ratio of 7/8, an original thin line is disconnected on the converted picture. More specifically, since the thin line determination condition of (P.sub.0.0)=(P.sub.2.0)=(P.sub.1.0) is established, it is determined that a vertical white thin line is present at the position of an original pixel P.sub.1.0, and the value of a converted pixel Q in FIG. 4(a) is set to be a white value. As the result, a converted picture becomes as shown in FIG. 4(b).
Therefore, the present invention relates to binary image reduction in a facsimile apparatus or the like and has as its object to provide a binary image reduction method which can reduce an image including an uneven pattern read from a scanner without being skipped or battered, and can reduce the image with high quality even if the reduction ratio is as small as, e.g., 1/4.